The purpose of this study is to determine if patients who develop a hyperosmolar syndrome suffer from temporary or permanent abnormalities in the regulation of thirst or the antidiuretic hormone, arginine vasopressin. Plasma vasopressin will be measured by immunoassay before and during treatment of the hyperosmolality and then again several months later during standardized tests of the osmoregulatory and baroregulatory systems. The effect of these tests on thirst also will be assayed. Analysis of the results will reveal if abnormalities exist in these patients and, if so, partially determine the nature and site of the lesion. The information will extend our understanding of osmoreceptor and neurohypophyseal function and also may be of help in preventing and/or treating the syndrome.